This invention relates in general to a probe holder and guide assembly for holding a measurement probe used in measuring a physical property of a workpiece and in particular to a probe holder and guide used in measuring coating thickness of a coated workpiece when the workpiece has a complex shape such as a jewelry item.
Instruments for non-destructively measuring the thickness of a coating on a substrate are known and have been widely used. These instruments utilize a probe which conventionally may either be hand held or mounted in a holder constructed to guide the probe under the control of the user into contact with the coating to be used. The probe may be, for example, a pencil-type model HH3 manufactured by UPA Technology, Inc., and includes a probe head which must contact the coating to be measured in order to made the measurement. With conventional apparatus, such as described in U.S. Pat. No. 4,155,009, beta backscatter techniques may be used to determine the coating thickness. With this apparatus, the probe head face is positioned in abutting relation with the coating and a measurement of the beta backscatter count is made during a measurement period to give an indication of the coating thickness.
One of the problems with hand held probes is the difficulty of positioning the probe head in relation to the coating surface such that repeatable measurements may be made. Typically, the measurement period is one-half minute and in general a user's hand is not sufficiently steady to maintain the same contact during the entire measurement period. Thus, error is introduced into the measurement process.
Probe holders were developed to hold the probe steady during the measurement process. An example of a conventional probe holder is described in U.S. Pat. No. 3,786,686 to William Hay et al. This holder includes a base on which the workpiece is mounted and means for holding the probe and for guiding the probe, under the control of the user, into contact with the workpiece. These holders maintain a constant contact between the probe head and the workpiece during the entire measurement period; however, they are most effective when the surface of the workpiece to be measured is flat. When measurements are made of more complex shapes, difficulties in obtaining accurate measurements are encountered because of the difficulty in properly positioning the probe head with respect to the coating surface such that repeatable measurements may be made. Attempts to modify the base holding the workpiece to accomodate these complex shapes, which may change from piece to piece, have not been successful and have not been practical.